A fuel cell comprises an electrolyte placed between an anode (fuel electrode) and a cathode (air electrode). It is a power generator, supplied with a fuel gas on the anode side and an oxidant gas on the cathode side to generate power by the electrochemical reactions between the fuel and oxidant via the electrolyte. A solid oxide fuel cell as one type of fuel cells efficiently generates power and, at the same time, allows fuel reforming reactions to proceed within the cell, because it operates at high temperature of 600 to 1000° C. It can potentially generate power at a lower cost than other fuel cell types, because it can be supplied with diversified fuels and work by a simple fuel cell system. It naturally discharges exhaust gases of high temperature, which can be reused more easily. Moreover, it can be easily combined with another system to form a cogeneration system, or with gas turbine to form a hybrid system.
SOFCs fall into two general categories by solid electrolyte shape, tube type and plate types. A tube type shape is more resistant to thermal stresses than a plate type, and this is a major advantage for an SOFC, which operates at high temperature.
However, a tube type shape involves a problem that it generally has a higher internal resistance than a plate shape, resulting from a longer current path in a tube type fuel cell (hereinafter referred to as tube type cell). Higher internal resistance causes problems of deteriorating cell characteristics, e.g., power generation efficiency and output density. Moreover, it also involves restrictions in connection of unit cells to each other, which makes it difficult to enhance volumetric energy density.
One prior art is Non-Patent Document 1 describing such a background.
Non-Patent Document 1: NEDO Report, 2002, by TOTO Co., Ltd.